The present invention relates to Schottky Barrier devices, and more particularly to Schottky Barrier devices which are readily integratable into integrated semiconductor circuits.
Surface barrier devices utilizing the Schottky effect based upon the rectifying characteristics exhibited by a metal-to-semiconductor interface are well known. Schottky Barrier diodes, also called "hot carrier" diodes have been recognized to exhibit fast switching speeds as well as relatively low forward barrier or turn-on voltages. Accordingly, the desirability of utilizing Schottky Barrier devices in integrated monolithic circuits has been recognized in the art. The two most desirable uses of Schottky Barrier diodes has been purely as clamps or shunts across PN semiconductor junctions as well as for storage purposes in Schottky Barrier diode monolithic memory array integrated circuits. The primary advantage of Schottky Barrier diodes over other diodes has been their relatively low forward barrier characteristics. Because of such low forward barrier characteristics, Schottky Barrier diode clamps may be used to prevent transistor saturation and thereby to provide faster turn-off time for digital circuitry, and faster switching speeds. Also, such diodes require less voltage when used in memory arrays, thereby having minimal heat and power dissipation problems.
One extensively used metallurgy for providing the ohmic contacts and interconnections in present integrated circuitry involves the use of a layer of platinum silicide in the contact holes making direct contact with the silicon substrate and a layer of aluminum over the platinum silicide. This layer of aluminum is coextensive with an aluminum layer pattern on the insulative layer over the semiconductor substrate which provides the interconnections. The reason that platinum silicide has been used in the contact holes is that aluminum has been found to make less than satisfactory direct ohmic contacts with silicon semiconductor substrates.
While such composite metallurgies of aluminum layers over platinum silicide have been extensively used in integrated circuits involving ohmic contacts and even suggested for usage in circuitry involving both ohmic and Schottky Barrier contacts (see U.S. Pat. No. 3,506,893), problems have been encountered and anticipated in the use of such a metallurgy in the more advanced integrated circuitry. Because such advanced circuitry requires greater device and metallization density, faster switching times and lower power dissipation, platinum silicide schottky Barrier contacts are considered to be less than satisfactory for the following reasons. First, a Schottky Barrier contact with silicon substrate has a forward barrier height in the order of 0.80eV (electron volts). This is a relatively high forward barrier height for a Schottky diode and will tend to decrease switching speeds. Of course, these higher barrier heights could be compensated for by increasing the contact area which would reduce the over-all voltage level required to render the Schottky Barrier diode conductive and thereby increase the switching speed. However, the relatively large area required for a plurality of such contacts would go contrary to the greater device and wiring densities required of advanced integrated circuits. Likewise, when used in Schottky Barrier diode memory arrays, the greater forward voltage requirements of the platinum silicide Schottky contacts are contrary to the direction in the memory array art of reducing the voltage requirements and thereby minimizing heat and power dissipation problems.
A possible solution for the problem presented by platinum silicide Schottky Barrier contacts in advanced integrated circuitry has been a metallurgical system wherein the metallurgy in the ohmic contacts and the Schottky Barrier contacts are formed in two separate steps. For example, the platinum silicide can first be formed only in the ohmic contact openings with the Schottky Barrier contact openings being masked off or closed during this step. Then, the Schottky Barrier contact openings are formed and aluminum deposited which forms the standard platinum silicide layer-aluminum layer metallurgy in the ohmic contacts and only an aluminum layer in the Schottky Barrier contacts. This does significantly reduce the forward barrier height of the Schottky contacts because aluminum has a forward barrier height in the order of 0.72eV. However, it does involve an additional processing step which the art would like to avoid if possible.